1. Field of the Invention
The present invention relates generally to a joint structure for an axle, and more particularly to a joint structure for an axle in a driving axle structure with a steering device.
2. Description of the Prior Art
According to a conventional arrangement, a driving axle structure equipped with a steering device includes generally a hollow spherical portion formed at the end portion of a non-rotatable tubular axle extending in the transverse direction of the car, a non-rotatable hollow axle casing disposed on the wheel side and a knuckle mounted rotatably on the spherical surface of the hollow spherical portion by a pair of king pins fitted to the end surface of the axle casing and positioned on the diameter substantially in the vertical direction relative to the hollow spherical portion and a king pin bearing supporting pivotally each of the king pins. A constant-velocity joint is disposed inside the space defined by the hollow spherical portion, the axle casing and the knuckle. Moreover, the outer race of the constant-velocity joint is coupled to a drive shaft disposed rotatably inside the axle casing on the wheel side while its inner race is coupled to an axle shaft supported rotatably inside the tubular axle on the car body side. In the driving axle structure having this conventional arrangement, a dust-tight annular seal is mounted to the knuckle in such a manner as to come into sliding contact with the spherical surface of the hollow spherical portion and to prevent any dust from entering the constant-velocity joint. According to this conventional arrangement, there is a limitation to the annular seal in that the seal can hardly be disposed ideally to the hollows spherical portion of the tubular axle because the tip of the knuckle, or in other words, the dust-tight seal, becomes an obstacle. The hollow spherical portion in the driving axle structure is a member which bears the load as part of the axle. For this reason, the conventional configuration of a rotary body housing an axle shaft which is worked by means of a lathe does not allow for any reduction in the diameter of the neck i.e. from the viewpoints of strength, so that a seal sliding surface necessary for satisfactory steering cannot be ensured and the maximum steering angle possible is therefore restrained up to about 30.degree..
A limitation imposed on the disposition of the constant-velocity joint is as follows: The tip of the hollow spherical portion formed at the end of the tubular axles butts against or interferes with the spherical surface of the outer race of the constant-velocity joint having an increased diameter so that the angle of rotation of the drive shaft disposed inside the axle casing and hence the angle of rotation of the knuckle relative to the hollow spherical portion, that is, the steering angle, is limited to approximately 30.degree.. In practice, however, the internal structure of the constant-velocity joint can permit the steering angle of up to about 40.degree., and an improvement is therefore desired in the disposition of the annular seal and the constant-velocity joint so as to proportionally increase the steering angle.
For the above-described reason, the prior art discloses an ellipitic-seal structure in which a surface of each knuckle on which a seal is mounted is inclined with respect to the hollow spherical portion of an associated tubular axle. This type of structure is disclosed, for example, in the specifications of Japanese Utility Model Publication No. 37386/1984 and 37387/1984.
First of all, a diagrammatic description will be made with respect to a front-wheel support device for a front drive vehicle described in Japanese Utility Model Publication No. 37386/1984. Either an oil seal member or an oil seal protection member is so formed as to have an axial length greater on the rear side than on the front side in the direction of movement of the vehicle when a knuckle housing is mounted. The seal of this prior art is disposed around a king-pin for pivoting the knuckle housing and a trunnion socket in an inclined manner at a predetermined angle, so that, when a vehicle is moved straight, a flat surface including the seal edge of either the oil seal member or the oil seal protection member is disposed closer to the shaft of the trunnion socket on the front side than on the rear side. In this front-wheel support device for a front drive vehicle, the seal is formed in a special shape so as to increase the steering angle. However, even if the seal is formed in such a special shape, it is not necessarily satisfactory in respect of its strength, sealing characteristics and durability. In addition, this type of seal is difficult to produce and of no practical use, thus leading to various problems.
In the second place, a diagrammatic description will be made with respect to an oil seal device for a front-wheel support of a front drive vehicle described in the specification of Japanese Utility Model Publication No. 37387/1984. Referring to an oil seal member in this prior art, a rubber-made seal body has an equal width along its entire circumference in a natural state, and a circumferential groove of a L-shaped in section is formed from a mounting flange of the seal body to an arm portion perpendicular to the mounting flange. A reinforced flange member of a metal-made cylindrical flange body has a cut surface on the cylindrical side, such cut surface being inclined with respect to a flange surface, and is adherently engaged with the circumferential groove in a state wherein this groove is under an elastic tension. The reinforced flange member is also disposed around a king-pin for pivoting the knuckle housing and the trunnion socket in an inclined manner at a predetermined angle, so that, when a vehicle is moved straight, a flat surface including the edge of the annual seal is positioned nearer the trunnion socket on the front side than on the rear side. As in the case of the above-described example, this oil seal device for the front-wheel support of a front drive vehicle needs a seal with a special shape, and thus there is a problem in that the seal provided may not necessarily be satisfactory from the viewpoint of strength, sealing characteristics and durability, nor with respect to considerations regarding the production of the seal.
Heretofore known drive axles equipped with a steering device include the arrangement which is opposite to the conventional arrangement described above or in other words, the arrangement wherein the disposition of the constant-velocity joint is reversed. This constant-velocity joint is disposed inside the space defined by the hollow spherical portion at the end of the tubular axle, the axle casing and the knuckle fixed to the axle casing. However, the inner race of the constant-velocity joint is coupled to the drive shaft disposed rotatably inside the axle casing on the wheel side while the outer race of the constant-velocity joint is coupled to the axle shaft supported rotatably inside the tubular axle on the car body side. This type of joint structure for an axle housing ballend is disclosed, for example, in the specification of Japanese Patent Laid-open No. 24226/1981. A follower steering shaft of the type disclosed in the above-mentioned specification will hereinafter be described with reference to FIG. 6.
Referring to FIG. 6, the follower steering shaft is essentially constituted by three components which can be separated from one another: a shank 71 coupled to a differential unit; a constant-velocity joint 72; and a steering shaft 74 for driving a boss or a sun gear (not shown) within a planetary unit. The constant-velocity joint 72 includes a spherical external coupling member 75 as an outer race mounted on one end of the shank 71 and an internal coupling member 77 as an inner race mounted on a corresponding end of the steering shaft 74. A bellows 73 hermetically seals the inner chamber of the constant-velocity joint 72, the bellows being detachably mounted on the external coupling member 75 of the constant-velocity joint 72, and a neck 76 of the bellows 73 being maintained in airtight contact with the steering shaft 74 for free movement along the axis thereof. The bellows 73 has an axial residual stress, and is therefore formed strongly enough to be extended to its maximum axial length. The constant-velocity joint 72 equipped with the bellows 73 can be removed without the need to release it. In such driving axles equipped with a steering device, however, bellows 73 as the dust-tight seal are interposed between a spherical external joint member 75 as the outer race of the constant-velocity joint 72 and a drive shaft 74, though the drive shaft 74 of the constant-velocity joint 72 having a reduced diameter is positioned on the wheel side, in order to prevent instrusion of dust and the like into the constant-velocity joint 72. Therefore, since the tip of the tubular axle 78 butts against or interferes with the outer peripheral surface of the bellows 73 having an increased diameter, the angle of ratation of the drive shaft disposed inside the axle casing and hence the angle of ratation of the knuckle to the tubular axle 78, that is, the steering angle, is limited in the same way as described above.